1. Field of the Invention
The present invention relates generally to light emitting diode array current power supplies and more particularly to such supplies which use an operational amplifier to regulate a constant current source.
2. Description of the Related Art
It has become desirable to employ non-impact xerographic-type printers for text and graphics. In such a printer, an electrostatic charge is formed on a photoreceptive surface of a moving drum or belt and selected areas of the surface are discharged by exposure to light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. The toner is then transferred to a sheet of plain paper and is heat-fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of non-impact printer employs an array of light emitting diodes (commonly referred to herein as LEDs) for exposing the photoreceptor surface. A row, or two closely spaced rows, of minute LEDs are positioned near an elongated lens so that their images are arrayed across the surface to be illuminated. As the surface moves past the line of LEDs, they are selectively activated to either emit light or not, thereby exposing or not exposing, the photoreceptive surface in a pattern corresponding to the LEDs activated.
To form good images in an LED printer, it is desirable that all of the light emitting diodes produce the same light output at the image plane when activated. This assures a uniform quality image all the way across a paper. The light output at the image plane depends on a number of factors including current, temperature, lens transmittance factors and processing parameters for forming the LED which may affect its light output as a function of current.
Light emitting diodes for print heads are formed on wafers of gallium arsenide or the like, suitably doped to conduct current and emit light. Long arrays of LEDs are formed on a wafer which is cut into separated dice, each having an array of LEDs. A row of such dice are assembled end-to-end to form a print head array. The light output of the LEDs on a given die are usually reasonably uniform, however, there may be variations from die to die as processing parameters differ between dice. There is some variation within dice from an individual wafer and greater variation from wafer to wafer.
The LEDs are driven by power supplies on integrated circuit chips. The current output of these chips may also vary depending on processing parameters in making these chips. Such variations may compound the variations in light output.
A parameter that is partly LED power supply dependent is the rise time for current flow. This is significant, since the exposure of the photoreceptive surface is a function of both intensity and illumination time. In an LED print head, there may be a few thousand LEDs across the width of the photoreceptive surface. The current in each LED may also be affected by the number of LEDs enabled at any time. Thus, there may be a relatively high current and concomitant higher light intensity or total exposure when a few LEDs are enabled, as compared with the current and light output when a very large number of LEDs are enabled.
Prior U.S. Pat. No. 4,864,216 to Kalata et al., which is incorporated herein by reference, provides a power supply for an LED print head in which a chip reference voltage biases a plurality of output driver FETs to provide substantially the same preselected current level to an LED associated with each output driver FET. The current is switched by a data signal applied to a data FET in series with each driver FET and LED. The Kalata et al. power supply represents a significant advance in the art in that it assures uniform light output across the array and a light output substantially independent of differences in the number of LEDs enabled.
It can be seen that in an ideal system, the current signal applied to each diode is a square wave of equal duration and magnitude. Such a signal generates uniform exposure of the charged surface, assuming equal light output for each LED at the image plane. The Kalata et al. power supply, while producing light substantially uniformly, tends to generate a current square wave with overshoot on the leading edge. This is a result of capacitive coupling between the gate and drain of the output driver FETs. When the data FET in a selected leg of the power supply switches on, the voltage on the drain of the output driver drops and tends to drag down the chip reference voltage applied to the gate of the output driver FET. This tends to turn on the FET harder thus providing more current until the capacitive coupling discharges. When such discharge occurs, the chip reference voltage returns to the selected level thereby dropping the current level through the output driver FET to the desired level.
As noted above, variations from one LED to another can cause variations in light output given the same current through each LED. Similarly, variations from chip to chip can cause differences in the average total chip light output given the same reference voltage on each chip. The Kalata et al. power supply suggests a variable resistor to change the chip reference voltage.
It would thus be desirable to compensate for variations from chip to chip and from LED to LED within a single chip. It would be advantageous to achieve such compensation with data programming signals.
It would also be desirable to provide a current through each LED which is substantially a square wave, i.e., without any, or at least without any consequential, overshoot or undershoot.
It would also be desirable to provide a light emitting diode array current power supply which can be powered by a lower current supply voltage than such prior art power supplies.